Interfacial assembly of binary atomic metal-Nx sites for high-performance energy devices

Anion-exchange membrane fuel cells and Zn–air batteries based on non-Pt group metal catalysts typically suffer from sluggish cathodic oxygen reduction. Designing advanced catalyst architectures to improve the catalyst’s oxygen reduction activity and boosting the accessible site density by increasing metal loading and site utilization are potential ways to achieve high device performances. Herein, we report an interfacial assembly strategy to achieve binary single-atomic Fe/Co-Nx with high mass loadings through constructing a nanocage structure and concentrating high-density accessible binary single-atomic Fe/Co–Nx sites in a porous shell. The prepared FeCo-NCH features metal loading with a single-atomic distribution as high as 7.9 wt% and an accessible site density of around 7.6 × 1019 sites g−1, surpassing most reported M–Nx catalysts. In anion exchange membrane fuel cells and zinc–air batteries, the FeCo-NCH material delivers peak power densities of 569.0 or 414.5 mW cm−2, 3.4 or 2.8 times higher than control devices assembled with FeCo-NC. These results suggest that the present strategy for promoting catalytic site utilization offers new possibilities for exploring efficient low-cost electrocatalysts to boost the performance of various energy devices.

For Co sites, all the elementary reaction steps on both FeN5-CoN4 and CoN4 models present a consistent downhill tendency at the U = 0 V, implying a spontaneous exothermal process. Upon increasing the thermodynamic equilibrium potential to 0.9 V, the ORR process on the FeN5-CoN4 model is still spontaneous, while the CoN4 possesses a slight endothermic process from the desorption of *OH in the fourth step, suggesting that the external force is needed to drive this process. At U = 1.23 V, the rate-determining step (RDS) in the CoN4 model is the desorption of adsorbed *OH with the largest Gibbs free energy change ΔG4 of 0.42 eV. In contrast, the corresponding ΔG4 in the FeN5-CoN4 model is much small (0.25 eV). The RDS for the FeN5-CoN4 model is the adsorption of the *OOH from the first electron transfer step with a ΔG1 of 0.29 eV. The limiting reaction energy barrier of ORR on FeN5-CoN4 (0.29 eV) is lower compared with the CoN4 model (0.42 eV). These results indicate that the introduction of the Fe site promotes the desorption of *OH on the Co site and optimizes the ORR pathway. and in FeN5-CoN4 models (blue line) at U=0 V, 0.9 V, and 1.23 V.
As for Fe sites, when U = 0 V, all the elementary reaction steps on FeN5-CoN4 and FeN5 models present a consistent downhill tendency, implying a spontaneous exothermal process. Upon increasing the thermodynamic equilibrium potential to 0.9 V, both the FeN5-CoN4 and FeN5 present the uphill free energy in the first electron transfer step (O2 + H2O + e -→ *OOH + OH -), which is the RDS for the two models. It is noted that the FeN5-CoN4 model gives a lower energy barrier of 0.49 eV than that of the FeN5 model (0.59 eV). Upon increasing the potential to 1.23 V, the first electron transfer step remains to be the RDS. The limiting reaction energy barrier of ORR on the FeN5-CoN4 (0.83 eV) model is still lower than that on the FeN5 (0.92 eV) model, implying the ORR on the Fe site in FeN5-CoN4 is energetically favorable to that in the FeN5. These results suggest that the introduction of the Co site significantly reduces the *OOH formation energy barrier on the Fe site and optimizes the ORR pathway.
Notably, the higher ORR performance of Co-NCH than Fe-NCH can be attributed to the following two reasons.
(1) Higher intrinsic activity of Co-N4 compared to the Fe-N5 sites. The DFT calculation results show that the ORR process is more favorable on the Co-N4 sites than that on the Fe-N5 sites. The RDS of the Co-N4 model is the last electron transfer step of the desorption of *OH with a free energy of 0.42 eV. While for the FeN5 model, the RDS is *OOH adsorption with a free energy of 0.92 eV, which is much higher than that of the CoN4 model. This result implies that CoN4 may possess a better ORR activity than FeN5 in our case.
(2) Co-NCH has more active sites than Fe-NCH. The metal loading of Co-NCH is 5.5 wt% vs. 2.4 wt% of Fe in Fe-NCH. The higher metal loading corresponds to more functional sites to catalyze the ORR. As shown in Supplementary Fig. 21, the Co-NCH exhibits a larger limiting current density than the Fe-NCH, indicating that more active sites participate in the ORR process than the Fe-NCH.

Supplementary Tables
Supplementary Table 1